With a battery system that employs a lithium battery, and in particular with a battery system for a vehicle, a method is known for performing diagnosis of over-charge and of over-discharge of the battery cells (for example, refer to Patent Document #1). Moreover, in relation to balancing circuits that are provided for reducing the differences in charge amounts between the battery cells, this battery system is also endowed with a function of diagnosing the operational state of such a system. Furthermore, a technique is known for detecting disconnections in connecting lines that between the battery cells and the circuitry for detecting the battery terminal voltages (for example, refer to Patent Document #2).